Molybdenum and tungsten oxide based gas sensors for high temperature detection of environmentally hazardous sulfur species

Sensors and Actuators B 237 (2016) 262–274

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Molybdenum and tungsten oxide based gas sensors for high temperature detection of environmentally hazardous sulfur species Engin C¸iftyürek, Katarzyna Sabolsky, Edward M. Sabolsky ∗ Department of Mechanical and Aerospace Engineering, West Virginia University (WVU), Morgantown, WV, 26505, USA

a r t i c l e

i n f o

Article history: Received 23 February 2016 Received in revised form 24 May 2016 Accepted 12 June 2016 Available online 16 June 2016 Keywords: Sulfur dioxide Hydrogen sulfide Tungsten oxide Molybdenum oxide High temperature gas sensors Strontium molybdenum oxide

a b s t r a c t There is an increasing desire to control and monitor gas emissions from coal-fired power plants and other industrial systems. With this desire, there is a growing need for distributed gas sensors to monitor these emissions at high temperature ( > 600◦ C), especially for pollutants such as SO2 and H2 S. The objective of this work was to investigate molybdenum and tungsten binary and ternary oxide thick films on a chemiresistive sensor platform for monitoring of gas sulfur species. The work evaluated the SO2 sensitivity of WO3 , MoO3 , SrMoO4 , NiMoO4 , Sr2 MgMoO6-␦ (SMM), Sr2 MgWO6-␦ (SMW), NiWO4 , and SrWO4 compositions at 600–1000◦ C. The SrMoO4 composition at both the micro- and nano-particulate scale showed the most promise in sensitivity, stability and selectivity to SO2 up to 1000◦ C. Hydrothermallysynthesized nano-SrMoO4 showed the highest sensor response with the Rmax values of −17.2, −50.2 and −40.5 upon exposure to a 20 min pulse of 2000 ppm of SO2 at 600◦ C, 800◦ C and 1000◦ C, respectively. Similar sensitivity trends were distinguished down to 1–5 min SO2 pulses. The nano-SrMoO4 showed low cross-selectivity to H2 and CO. Finally, the nano-SrMoO4 sensor was also tested with H2 and coal syngas containing 5–100 ppm H2 S, where high sensitivities were realized for both, but the sensing mechanism was altered in the latter (n-type to p-type semiconducting behavior). In order to better understand the sensing mechanism, extensive microstructural, electronic and chemical property characterizations were completed in this work. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Sulfur dioxide (SO2 ) is known to be a major air pollutant contributing to acid rain and acidic particulates, which are dangerous not only for human health but also a major contributor to greenhouse effects [1]. Coal contains a significant level of sulfur species, and with the increased burning of this fossil fuel for energy by developing countries, the release of sulfur species into the atmosphere is expected to increase dramatically over the next decade [2,3] Coal-fired power plants are the main emitter of sulfur in the form of SO2 (and hydrogen sulfide, H2 S) [4,5] Modern desulphurization methods can eliminate 98% of the SO2 from the emission gases by applying multiple levels of filtering, where the prime is through passing the exhaust gas through lime. However, SO2 emission control has become a significant challenge for scientists and engineers [6] With this in mind, it is very important to develop and deploy sulfur sensor systems to monitor and quantify the level of SO2 /H2 S from these energy and industrial systems. The key factor

∗ Corresponding author. E-mail address: [email protected] (E.M. Sabolsky). http://dx.doi.org/10.1016/j.snb.2016.06.071 0925-4005/© 2016 Elsevier B.V. All rights reserved.

is to develop sensors that are capable of withstanding harsh environmental conditions that exist in coal processing, which would enable the sensors to be placed at any location within the exhaust stream. For this to occur, the sensors must be robust, catalytically active to SO2 /H2 S, and display low cross-selectivity towards other reducing gases, and this must be simultaneously achieved at high temperatures. Many SO2 sensors base their functionality on the use of a liquid electrolyte or a polymeric material [7–10]. These sensing technologies are generally limited to low temperatures. The measurement efficiency and accuracy would be increased if sensing could be completed near the combustion or reforming event at high temperatures (>800◦ C). The use of solid-state sensors has also been discussed in literature, and these primarily focus on the use of semi-conducting metal oxide compositions to sense SO2 . The number of these publications is limited in comparison to the sensing of other gasses using these oxides. Semi-conducting metal oxides that were investigated for SO2 sensing include CeO2 , WO3 , V2 O5 TiO2 , MoO3 -SnO2 and NiO [10–16]. Generally, the semi-conducting metal oxides used for SO2 sensing were only applicable at low temperature (<500◦ C) due to stability issues in the highly reducing atmospheres [8] There are only a few instances of materials

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that were tested at higher temperatures (>300◦ C) for sulfur species; four studies deserve to be mentioned in detail due to their unique strategies employed in order to increase the sensor response and selectivity. Liang et al. modified a compact tubular sensor used in a potentiometric sensor design, where they used a V2 O5 -modified NASICON membrane with a vanadium (V)-doped TiO2 composition acting as the SO2 sensing material [11,12] The sensor showed excellent sensor response to SO2 at 300◦ C. Beyond this temperature, the sensing capabilities decreased and diminished drastically beyond 500◦ C. Morris et al. similarly investigated the sensing capabilities of various level V-doped TiO2 against different levels of SO2 balanced with dry air (with nearly 20% O2 ) in a resistive-type sensor design. The maximum sensor response for a 0.5% V-doped TiO2 composition was −60% at 375◦ C upon exposure to 1000 ppm SO2 with a 16.5 min response time. An increase in the temperature decreased the sensor response and eventually the response diminished down to only −10% at ∼600◦ C [11] The negative designation for the sensor response indicates that the response was n-type in behavior, where the resistance decreased on exposure to the sulfur species. It is noteworthy to indicate that TiO2 is the most successful semi-conducting metal oxide for detecting SO2 at high temperature. Regrettably, the response diminishes as the temperature increases beyond 600◦ C. CeO2 is the last example of a semi-conducting oxides tested and tailored for SO2 sensor applications at high temperature within a resistive-type sensor architecture. Varhegyi et al. concluded that the resistance of the CeO2 is not affected by the presence of SO2 up to 500 ppm, unless the O2 partial pressure is low (between 300 and 800◦ C) due to fact that physi/chemi-adsorption does not occur due to the defect state of the CeO2 surface [17] In other words, the presence of chemically or physically adsorbed oxygen reduces electron exchange reactions with SO2 on the CeO2 surface. These authors basically developed a better understanding of the SO2 CeO2 surface and bulk interactions, and proposed several mechanisms regarding surface CeO2 and SO2 reactions. In the case of H2 S sensing, the volume of compositions tested is much larger than that of SO2 . Although the volume of the literature is remarkable, most of the works are devoted to semiconducting oxides and the operation temperatures of these sensors have been reported from only room temperature to 300◦ C. One of the few papers describing sensing H2 S at higher temperatures was completed by Dawson et al.; they utilized a Cr2-y Tiy O3+x sensing composition for a resistive-type sensor design [18] The material demonstrated a p-type characteristic at elevated temperatures (>250◦ C) and showed an increase in resistance upon exposure to H2 S (50 ppm) within a testing range of 250–500◦ C. It is the sole paper, at least to our knowledge in literature, that provided temperature desorption curves for both SO2 and H2 S. It was seen that H2 S exhibited two maxima at about 150◦ C and 470◦ C; however, the loss of SO2 from the surface occurred at 470◦ C. It was concluded that a sensor that operates at 350◦ C can be cleaned by heat treatment, and a pre-treatment will increase the sensor response of the sensor [18] Some of the other transition metal oxides demonstrated for H2 S sensing are as follows: PdOx , WO3 , MoO3 , In2 O3 , CeO2 , SnO2 , TiO2 , ZnO, CuO, CdO, and various ferrites [10,15,19–26] A majority of these reports are based on WO3 compositions [8,10,15,19–21,22–34] In the current work, tungstate and molybdate compositions were investigated as alternative sensing materials to the typical binary compositions for sulfur gas species at higher testing temperatures (≥600◦ C). Tungstates and molybdates are known to be wide band gap oxide semiconductors (3–5 eV). These compositions also demonstrate sulfur activity and present adequate stability at higher temperatures (especially compared to that of WO3 and MoO3 ). There are currently no publications focusing on these compositional families for sulfur species. As stated above, there are examples of the use of binary tungsten and molybdenum oxide

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compositions for sulfur sensing. In the case of SO2 sensing, the WO3 composition was tested by many researchers with various morphologies as well as deposition techniques [7,10,35–42] Unfortunately, the same composition was shown to also be sensitive to O3 , NO, NO2 , NOx , H2 , DMA, NH3 , C2 H5 OH, and O2 species. WO3 is a d0 compound in its stoichiometric form [43]; therefore, it exhibits an insulator behavior. However, the non-stoichiometric forms of tungsten oxides (WO3-x ) show an n-type semi-conducting behavior with a band gap of 2.7 eV. Among the wide range of reducing and oxidizing agents tested, it was observed that NOx , H2 S and SO2 were detected with high response compared to the other gasses. In the case of the binary MoO3 composition, it is structurally similar to that of WO3 with similar electronic characteristics. The material is an n-type semi-conductor with a band gap of 3.2 eV, but it possesses a low melting point (795◦ C). The low melting temperature restricts use of this composition at high temperature. MoO3 was first tested for sensing NH3 , CO, CH4 , NO2 and SO2 by Azad et al. They used the composition alone and as a composite with ZrO2 [44–46]. The composition showed adequate response to all gasses; unfortunately, it displayed a high cross-selectivity to H2 . Imawan et al. was one of the few other researchers that investigated gas sensing properties of MoO3 (modified with V2 O5 ) for SO2 gas, where the sputtered films showed 500 times higher response to H2 over that of NH3 and SO2 . Furthermore, the response to H2 was 12 times that of SO2 [45,46] The ternary and quaternary tungsten and molybdenum oxides used in this work were tested initially within a resistive-type sensor platform (also termed as chemi-resistive platform) for impurity levels of SO2 and H2 S in the range of 5–2000 ppm. As known by many researchers, the selective detection of SO2 and H2 S in the presence of reducing gasses, such as H2 and CO, is a formidable task, because the working mechanism of the resistive-type sensor is based primarily on surface redox (reduction-oxidation) reactions. These reducing gasses have an affinity for oxidation on oxide surfaces, which usually alters the surface defect state and carrier concentration, and thus overshadows any electrochemical reactions occurring with other interested species (such as SO2 and H2 S in this case). The SO2 sensing tests were initially completed for all sensing materials; the most sensitive compositions towards SO2 were then further tested for 50–100 ppm H2 S within N2 and coal syngas gas streams. After this evaluation process, H2 and CO crossselectivity tests were completed in the range of 1000–4000 ppm in order to evaluate the level of interference to the SO2 and H2 S sensing for only the down-selected sensing materials. Within the work, multiple compositions were initially tested for comparison purposes in the micron-size range (either synthesized by solid-state method or commercially purchased). The best performing composition was then synthesized to the nano-scale by a hydrothermal route. The compositions of interest were: WO3 , MoO3 , SrMoO4 , NiMoO4 , Sr2 MgMoO6-␦ (SMM), Sr2 MgWO6-␦ (SMW), NiWO4 , and SrWO4 .

2. Experimental 2.1. Material source for initial sulfur sensing evaluation As will be discussed in the “Results and Discussion” section, commercially available and solid-state powders (which all displayed an average particle size between 1 and 10 ␮m) were initially screened for their sulfur sensing capabilities using 2000 ppm SO2 (balanced with N2 ) at temperatures of 600–1000◦ C. The following materials were initially tested: MoO3 (molybdenum oxide, 99.5%, Alfa Aesar), WO3 (tungsten oxide, Alfa Aesar, 99.8%), SrMoO4 (Alfa Aesar, strontium molybdenum oxide, 99.9%), NiWO4 (nickel tungsten oxide, 99.9%), SrWO4 (strontium tungsten oxide, 99.9%) and

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NiMoO4 (nickel molybdenum oxide, 99%, Alfa Aesar). The MoO3 and WO3 compositions were included in this work in order to provide a baseline performance and stability level to compare to the ternary and quaternary compositions tested at the higher temperatures and lower oxygen partial pressures. These conditions were not reported by other researchers in literature. For the Sr2 MgMoO6-␦ (SMM) and Sr2 MgWO6-␦ (SMW) compositions, commercial powders were not available, so a basic solid-state method was employed to synthesize the powders. The precursor materials used were SrCO3 , MgO, and MoO3 (Alfa Aesar, 99.9% purity for each). The powders were ballmilled in ethanol and then calcined at 1000◦ C for 4 h in air within a high-form alumina crucible. The powder was then attrition-milled for 4 h to the specified particle size range. The material was re-fired at 1200◦ C for 4 h in 5% H2 /N2 to insure full dissolution of the residual SrMoO4 , which is typically found to remain as a second phase within the powder after the first firing [47,48] 2.2. Synthesis of molybdate sensing nanomaterials The best performing sensor material from the initial 2000 ppm SO2 screening tests was re-synthesized to the nano-scale using a hydrothermal method in order to increase the active sensing surface area. The best performer was the SrMoO4 composition. The sensor testing protocol for this nanomaterial will be discussed further in the next subsection. To control the particle size and morphology of the nanomaterial, the hydrothermal solution variables were altered, such as pH, ion concentration, and the processing temperature (and time). The method was based upon the previous works discussed in literature for SrMoO4 , which produced varying nano-particle morphologies [49–53] The general procedure includes the dissolution of 1.48 g of strontium nitrate (Sr(NO3 )2 (99.0%,) and 1.23 g of ammonium molybdate (((NH4 )6 ·Mo7 O24 )·4H2 O, 99%, Alfa Aesar) into de-ionized (and de-carbonized) water in separate beakers. A clear solution was obtained after magnetic stirring the two solutions for 15 min. The pH of the final solution ranged from 4.5 to 5.5. The final product was transferred into a 300 ml PTFE lined autoclave and processed at 180◦ C, 120◦ C and 80◦ C for varying from 8 h to 12 h, with a 3◦ C heating and cooling rate in all cases. After mixing the two solutions, the final pH of the solution was adjusted to the corresponding values through the addition of ammonia hydroxide. The effect of altering the chemistry and thermal processing on the morphology and size will be further discussed in the “Results and Discussion” section. The as-synthesized SrMoO4 powder was washed with de-ionized (and de-carbonized) water in order to remove remaining salts. The washing procedure was repeated until the conductivity (␴) of the solution measured less than 10 mS·cm−1 . 2.3. Sensor fabrication and testing protocol The sensor platform was composed of alumina (Al2 O3 ) substrates which were fabricated by a tape-casting/lamination process. The green laminates were fired to 1400◦ C for 2 h, and were diamond polished to a 1 ␮m finish. The platinum (Pt) interdigitized electrodes (IDEs) were screen-printed through 100 ␮m mesh screen and annealed at 1200◦ C for stabilization. The Pt ink was made from Pt powder (Technic Engineered Powders) with average particle size of around 1 ␮m. The electrodes displayed a total length of 10 mm with finger widths and finger spacing of 0.25 mm. The sensor materials were dispersed within an ink vehicle (J2 M, 63/2, Johnson-Matthey, UK) and then stenciled over the IDEs. The sensing material was printed to a thickness of ∼300 ␮m and sintered at 1200◦ C in order to promote the adhesion between the substrate and printed sensing material. The sensors were placed on a hotplate at 90◦ C for half an hour to dry and then heat treated in a box-furnace with a heating rate of 2◦ C·min−1 to 600◦ C, and then

4◦ C·min−1 to 1200◦ C, and held for 1 h. A tube furnace was used as the testing chamber, where the Pt electrode wires from the sensor were connected to the sample in the center of the tube through a sealed two-bore holed alumina rod. The sensor face was placed horizontal to the testing gas flow. The testing gas composition was controlled by mass flow controllers (Sierra Instruments, MFCs) (for each gas) and the 2-point resistance signal was measured with a Keithley 2700 Multimeter/Data Acquisition System which supplied a 5–10 ␮A DC current to obtain the measurement. A LabView® program controlled the testing and data acquisition in order to simultaneously read gas flow rate, temperature and resistance of the sensor during testing. Parameters such as sensor response, cross-selectivity, and response and recovery times were characterized. The relative resistance change (R) is defined as a change in the resistance of a SMO upon exposure to target gas. It is calculated with the formula presented in Eq. (1) for an n-type material. R represents relative change in resistance, in where R0 is the resistance in 1% O2 balanced with pure N2 and RE is the resistance during exposure to the target gas. Relative resistance change (R) =

 R − R  E 0 R0



× 100

(1)

The response time was defined by the time for the sensor to reach 90% of the total resistance change due to the change in concentration of the target gas [54–59] If the relative change in R is a negative value, then response will be termed in this paper as an “n-type response”. In the case that the R is a positive value, then the response will be termed as a “p-type response”. The absolute maximum of relative resistance change will be termed as the Rmax throughout the work. The gas exposure cycle used throughout the work are presented as insets within the figures, which includes the concentration of the target gas and time of exposure during holds at the designated temperatures. In all tests, the total flow was adjusted to 50 sccm by adjusting the flow of H2 , CO, SO2 , N2 and O2 via the mass flow controllers (MFCs). Three different concentration levels of target gas, balanced with nitrogen (N2 , 99.99%, Matheson Ultra High Purity grade), were tested at three different exposure times. During these tests, the sensors were heated and held at 600◦ C, 800◦ C, and 1000◦ C for 8.5 h, and then the sensors were cooled to room temperature under atmospheric conditions, unless otherwise indicated. In a few situations, the sensor was cooled under N2 flow in order to conduct chemical analysis of the final sensing material. Three different ultra-high purity O2 concentrations (1, 5 and 20%) were further tested for the selected compounds in order to understand the effect of possible oxygen partial pressure variations. Matheson research grade 99.998% O2 was used in this work. As seen in the inset plots for SO2 testing, the exposure cycle increased the ppm level of SO2 from 500 to 1000, and then to 2000 ppm, and then decreased back down to 500 ppm. Each of these gas concentrations were held for 20 min and then the pyramid was repeated with a hold time of 5 min. A final pulse of the maximum concentration was placed on the sensors for 30, 15, and 5 s in order to test the response and recovery time. Each of these pulses were balanced with pure N2 with 1% O2 background during the entire isothermal hold. A 30 min holding time in the 1%O2 + N2 mixture was placed on the sensors between each pulse in order to allow the sensors to recover before further exposure testing. As previously discussed, the sensing materials showing the highest sensor response to SO2 were selected and tested in the presence of 1000, 2000 and 4000 ppm for H2 and CO. The material that showed the highest sensor response towards SO2 was further tested for 50–100 ppm H2 S and also tested within a coal syngas composition. The coal syngas composition selected was 32% H2 , 40% CO, 20% CO2 , 1% O2 , 1% steam (H2 O), 5% N2 and 1% H2 S. The latter two testing practices differ from the initial SO2 tests, and those differences will be explained in detail in the related subsections [60,61].

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Fig. 1. Rmax values for the various molybdenum and tungsten oxides.

2.4. Sensing material characterization The crystallinity of all synthesized powders were characterized by X-ray diffraction (XRD) at room temperature using a Panalytical X-Pert Pro diffractometer (PW 3040 Pro) with Cu K␣ radiation with a scan rate of 2◦ min−1 . Transmission electron microscope (JEM-2100 TEM) was utilized to confirm crystallinity and lattice parameter. Their morphologies were examined using a scanning electron microscope (JSM–7600 F SEM). The energy dispersive spectroscopy (EDS) was completed using an Oxford INCA 350 connected to JEOL 7600 F SEM. X-ray photoelectron spectroscopy (XPS PHI 5000 Versaprobe) was also conducted to further analyze the secondary phase formation and/or chemical state of the ions within the structures. The X-ray source was operated at 15 kV and 25 W using Al K␣ (1486.6 eV) radiation. The powders were analyzed by a combination of 117.40 eV survey scan and 23.50 eV detailed scans of the relevant peaks. A 0.5 eV step was used for the survey scan and a 0.05 eV step for the detailed scan. Prior to spectral analysis, all coating surfaces were cleaned to remove atmospheric and postdepositional contamination with Ar ion sputter cleaning at 2 kV accelerating voltage for 30 s. During the measurement, the analysis chamber pressure was maintained at (∼10−11 Torr). Gold (Au) was used as a reference with the binding value of 84.5 eV when possible; otherwise, carbon (C) was used as the reference at a value of 284.5 eV. In addition to this characterization, the nano-SrMoO4 material was subjected to further UV–Vis (ultraviolet-visible spectroscopy) in order to measure the band-gap (Eg ). 3. Results and discussion 3.1. Initial screening of SO2 sensing of the molybdates and tungstates In this subsection, the initial screen testing of the molybdate and tungstate systems will be briefly discussed. Again, the compositions tested were WO3 , MoO3 , SMM, SMW, NiMoO4 , NiWO4 , SrWO4 and SrMoO4 , where the particles were in the micron-size range (1–10 ␮m for all compositions). Fig. 1 shows the comparison of the Rmax values for the different sensing materials at three different temperatures against 2000 ppm of SO2 for 20 min pulses balanced with N2 + 1% O2 . WO3 showed an n-type sensor response towards SO2 at 600◦ C with a Rmax of −15; however, at elevated temperatures (800◦ C and 1000◦ C), the response decreased and eventually diminished. Chemical reduction of the oxide semiconductor was suspected to

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be the reason behind the decrease in the magnitude of the response. Therefore, XPS analysis (not included) was conducted in order to clarify the final chemical state of the WO3 after the complete sensor test. Cooling the WO3 material post-testing in a N2 atmosphere further clarified the final phase state by showing that the WO3 reduced to metallic tungsten (W) during testing. The W was the main photoelectron spectra shown for a large portion of the surface, which indicated that much of the WO3 reduced to the metallic state. The W metallic phase 4f peaks were observed at 31.6 and 33.7 eV with 2.1 eV spin-orbital splitting. Molybdenum trioxide (MoO3 ) has previously not been studied to a large extent for gas sensing applications. As seen in Fig. 1, the sensor initially showed a large p-type response for MoO3 , which is unusual for the n-type semiconductor MoO3 . The Rmax was 14 in this case. It should be noted that after the full test, it was observed that the MoO3 sensing material was almost fully evaporated due to its high vapor pressure. The reason is not clear for the p-type response, and no literature information could be found in order to further clarify the response. However, it is well-known for WO3 that a change in response behavior (from n-type to p-type or opposite) has occurred upon exposure to different levels of target gasses at different temperatures and oxygen partial pressures [39]. The same characteristic can also attributed to MoO3 in order to explain the unexpected p-type response. The Sr2 MgMoO6 (SMM) and Sr2 MgWO6 (SMW) compositions did not sense SO2 at 600◦ C. The Rmax values for 2000 ppm of SO2 were −21 and −20 at 800◦ C, and similarly, −20.5 and −21.5 at 1000◦ C for SMM and SMW, respectively. The SO2 sensor response of the SMM and SMW were comparable to each other. Both compositions were inaccurate at distinguishing different levels of the target gas. SMM and SMW were not further investigated due its lack of repeatability and lower response value; moreover, it showed cross-selectivity against H2 at 1000◦ C (not included). Although the performance was not optimal, the SMM and SMW showed superior high temperature sensing compared to binary oxides of tungsten and molybdenum due to their high temperature stability. As can be seen from Fig. 1, NiMoO4 showed a p-type response with a Rmax value of 20 at 600◦ C upon the exposure to 2000 ppm SO2 ; however, repeatability was very poor at this temperature. At 800◦ C, the sensing material experienced a large reduction event. At 1000◦ C, the recorded resistance value was comparable to the metallic state of Ni. Due to this complete reduction event, the material was not further subjected to the investigation and experiments were not completed. The XPS analysis proved that the tested sensor surface was composed of Ni metal and different sub-oxides of Ni, such as Ni2 O3 , NiO, and metallic Ni over the sensor surface which dictated the response behavior. The metallic nature of the sensor material eliminated the desired electrocatalytic reaction which allows for sensing of the target gas. The NiWO4 exhibited an n-type response with a Rmax value of −5 and −16 for 2000 ppm of SO2 at 600◦ C and 800◦ C, respectively (as shown in Fig. 1). The magnitude in sensor response did not further increase above 800◦ C; it decreased to −2.5 at 1000◦ C. At this temperature for the 2000 ppm of SO2 , the sensor response was not smooth as observed at 800◦ C; moreover, at 1000◦ C the signal was not reliable and diminished towards the end of testing. The SEM micrographs of the material after testing showed small grains coating over the larger NiWO4 (not included in this work). The XPS characterization of the sensor surface was again completed after SO2 testing (and cooled down in N2 ) (not included in this work). The XPS spectra showed the presence of metallic Ni on the surface. The detailed spectra taken from the sensor showed Ni 2p3/2 and 2p1/2 doublets, in which the main photoelectron lines are positioned at 852.8 and 870.1 eV, respectively. The binding energy values agree well with chemical state values for metallic Ni at 852.7 ± 0.1 and 869.9 ± 0.1 eV. Also, NiO (Ni2+ ) was also identified within the mate-

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Table 1 Process parameters for hydrothermal synthesis of nano-SrMoO4 with different morphologies. Morphology

Nano-teeth Nano-sheet Nano-flowers Micro-rice

Processing Time (h) and Temperature (◦ C)

12 & 180 12 & 120 8 & 80 8 & 80

Precursor ion concentration (M)

pH

Anion (MO2− ) 4

Cation (Sr2+ )

0.0134 0.025 0.05 0.05

0.0934 0.175 0.35 0.35

8 8 8 10

Fig. 2. SEM micrographs of the hydrothermal synthesized SrMoO4 nano (a)-teeth (b)-sheet (c)-flowers and (d) micro-rice.

rial, but the amount was lower compared to metallic Ni. As in the case of NiMoO4 , the NiWO4 structure was shown to reduce at 1000◦ C, even though the atmosphere was not strongly reducing. Ni migrated and subsequently agglomerated on the surface of the grains. This fact was clearly observed from the XPS data of the Ni and W. This accumulation of Ni grains on the surface was also observed by Sabolsky et al. [62], where the same composition was used within a solid-oxide fuel cell anode. Basically it can be summarized, NiWO4 showed high sensor response for SO2 up to 800◦ C and showed a lower reduction rate compared to the WO3 baseline composition in the same atmosphere. Above 800◦ C, metallic phases were formed which drastically decreased the sensor response. The inclusion of Ni within the molybdate and tungstate compounds is a good choice for SO2 adsorption, but it is not good for redox stability, especially beyond 800◦ C under an insufficient oxygen background concentration. The maximum relative resistance change (Rmax ) was measured for SrWO4 at three different temperatures, and this again is shown in Fig. 1. The compound has a scheelite crystal structure and was recently utilized within various photocatalysis processes [63] At 600 ◦ C, the sensor response was p-type upon exposure to SO2 . The Rmax of SrWO4 toward SO2 was 110. At 800 ◦ C, the sensing behavior of SrWO4 changed to an n-type response, where the Rmax was −18. The response also showed great variance and began to drift over time. Moreover, the sensor did not recover after removal of the SO2 before moving to the 1000◦ C test temperature. The change in the sensing response mechanism has been seen before for WO3 based sensors, but p-type to n-type (or opposite) was only reported for a change in oxygen content and/or temperature. XPS analysis

of the SrWO4 sensor after SO2 testing (and cooled down in N2 ) was also completed (not included in this work). Broad W, O and Sr peaks indicated that the SrWO4 structure reduced during testing; however, metallic W0 was not detected which is important to note. Moreover, the Sr was also found to be in two different chemical states (but both of them oxides). This occurrence may be due to the partial reduction of the SrWO4 and the formation of WS2 , which is a p-type semiconductor. The broad XPS peak, which is supposed to be a doublet with the clear electron orbital splitting, may correspond to the presence of O in different oxidation states coordinated with the W and Sr. Even with the alternation in sensing compound chemistry, the functionality of the material survived, and no adsorbed sulfur species was detected. At 1000◦ C, the SrWO4 showed repeatable and smooth responses to SO2 characterized by an n-type behavior with a Rmax value of −20. Fig. 1 provides the Rmax values of the SrMoO4 against 2000 ppm of SO2 at the three different temperatures. The SrMoO4 showed n-type response at all temperatures. It showed the highest sensor response toward SO2 among all of the initial micron-size oxides tested, and interestingly, the Rmax did not vary too much for the same 2000 ppm level at the three different temperatures. The Rmax values were −31, −38 and −28 at 600◦ C, 800◦ C and 1000◦ C, respectively. The response was repeatable. Due to this promising performance, the SrMoO4 composition was synthesized by a hydrothermal process (described in the “Experimental Section”) at a finer, nano-scale for further testing. The nano-flower morphology for the nano-SrMoO4 was tested to the 2000 ppm SO2, similar to the previously described compositions. As shown in Fig. 1, the Rmax values of the nano-SrMoO4 were −17.2, −50.2., and −40.5 at 600◦ C,

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Table 2 Comparison of calculated cell parameters and reference data (01-085-0586). In this study

In JCPDS no. 00-085-0586

/ c, ␣=ß = ␥=90◦ , Space group: I41/a, Space Group number: 88, Scheelite Crystal system: Tetragonal unit cell, a = b = (112) plane at 27.892◦ , d(112) = 3.1988 Å (204) plane at 45.302◦ , d(204) = 2.0008 Å (004) plane at 29.907◦ , d(004) = 2.9989 Å a = b = 5.3236 Å, c = 12.123 Å, a/c = 0.4391, c/a = 2.2774 Crystalline Size with Scherrer formula: 35 nm

(112) plane at 27.667◦ , d(112) = 3.2205 Å (204) plane at 45.134◦ , d(204) = 2.0072 Å (004) plane at 29.706◦ , d(004) = 3.0050 Å a = b = 5.3944 Å, c = 12.020, a/c = 0.4487, c/a = 2.2286 NA

800◦ C and 1000◦ C, respectively. Nano-SrMoO4 showed the highest sensor response toward SO2 at 800◦ C and 1000◦ C compared to all other compounds tested. Therefore, in the rest of the present work, further testing and characterization was completed on the nanoSrMoO4 composition to better understand the sensing mechanism, as well as, the compound’s physical and chemical characteristics in the synthesized and post-mortem testing state. 3.2. Characterization of nano-SrMoO4 In this section, the characterization results for the nano-SrMoO4 powders fabricated by various hydrothermal routes will be presented. As discussed in subsection 2.2, the thermal conditions, pH and solids loading were all altered in order to control the morphology of the nanomaterials. Table 1 shows the conditions used for the synthesis of the nano-SrMoO4 compositions. Fig. 2 displays the micrographs of the as-synthesized nanomaterials, where various morphologies, termed as nano-teeth, −sheet, −flowers and micro-rice throughout the paper, were fabricated. The decrease in the temperature and increase in the ion concentration affected the growth rate of the nano-particulates, and this decreased the feature size as expected [53]. As can be seen from Fig. 2-c and -d, at the same pH and higher solute concentration with lower processing time and temperature, particles with a higher anisotropic morphology were synthesized. Decreasing the hydrothermal processing temperature and doubling the anion and cation concentration lead to more isotropic growth. The morphology changed from nanoteeth to nano-sheets with symmetrical spatial growth. It should be noted that the same processing time and temperature were used, but the different pH levels caused changes in the growth mechanism and rate. The morphology of the powder evolved from a nano-flower-like morphology (with a loosely packed anisotropic structure) to a densely packed, isotropic rice-shaped morphology. The SrMoO4 -nanoflower morphology was chosen for future sensing experiments due to its loosely packed structure with increased surface area (and thus, potential sensing efficiency). The SrMoO4 nanoflowers will be termed as nano-SrMoO4 within the rest of the paper. All of the as-synthesized nano-structures for the SrMoO4 material were subjected to XRD analysis to investigate the crystal structure of each powder. Fig. 3 displays the XRD spectra of the as-synthesized nano-SrMoO4 . All of the diffraction peaks can be indexed with the JCPDS card number 01-085-0586. The sharp and high intensity diffraction peaks of the as-synthesized nano-flower powder showed very good crystallinity with no secondary phases such as MoO3 and/or SrO present; therefore, further calcination steps were not needed after the hydrothermal technique to form the full phase materials. The nano-sheets, nano-teeth and nano-rice morphologies also showed an excellent match between the measured and reference XRD data (not included in this work). The XRD spectra showed that the anisotropic tetragonal structure was synthesized for all SrMoO4 morphologies. This form of SrMoO4 displays the scheelite structure which contains four formula units per unit cell. A unit cell contains 4 Sr2+ and four (MoO4 )2− . In the scheelite structure, Mo is surrounded by 4O2− and the Sr is surrounded by

Fig. 3. XRD spectrum of the nano-SrMoO4 with the HR-TEM image (inset) of the (112) plane for the as-synthesize nano-flowers.

Fig. 4. XPS core spectrum of Sr, O and Mo in the as-synthesized nano-SrMoO4 .

8O2− . Investigating the nano-flower morphology specifically, the XRD pattern matched well with the JCPDS 01-085-0586 data, with ´˚ Using the the lattice parameters a = b = 5.3236 and c = 12.1230 A. plane spacing and Bragg‘s law for diffraction, the lattice parameters were calculated and presented in Table 2. As compared to the position given in the JCPDS 01-085-0586, the calculated a-, band c-parameters of the as-synthesized nano-SrMoO4 nanomaterial showed residual strain, which is shown by the constriction of the a- and b-lattice parameters and the expansion in the c- lattice parameter. The strain may be attributed to oxygen vacancies (VO¨ ) in addition to charge compensation and cation/anion columbic interactions. The strain is further supported by HRTEM (High Resolution Transmission Electron Microscopy) imaging of the (112) plane of the as-synthesized powder. As seen in this micrograph, ´˚ the measured d-spacing for (112) via HRTEM was 3.20 A. A detailed XPS analysis was also conducted in order to quantify the chemical state of the Mo and Sr in nano-SrMoO4, in addition to the defect concentration within the structure (such as oxygen  vacancies (VO¨ ) and oxygen interstitials (Oi )). Fig. 4 presents the deconvolution of the O, Sr and Mo main photoelectron lines that were used to quantify the relative amount of the different chemical states. Oxygen peaks were consistently fitted to a nearly Gaussian

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distribution, which assisted in assigning the relative contribution of the different chemical states of the oxygen. After deconvolution of the O 1s spectrum, three main chemical states of the oxygen were determined. Those are located at 530.34, 531.15 and 532.85 eV. The former is in a good match with the values presented in literature for lattice O2− ion in SrMoO4, while the latter corresponds to that reported for interstitial oxygen ions; the last photoelectron energy aligns with the values reported for chemisorbed oxygen (surface adsorbed oxygen) [64–70] The corresponding full width at half maximum (FWHM) values for lattice, interstitial and adsorbed oxygen ions are 1.2, 1.9 and 1.8 eV, respectively. The broad nature of the interstitial component can be interpreted as the contribution of the oxygen deficient regions [69] In this work, 50 at% of O2− ions were located at regular lattice sites, while 37 at% of O2− ions was positioned at interstitials, with the remaining 13 at% was detected as chemi/physi-adsorbed. The Mo 3d doublet consisted of two distinct chemical states, Mo6+ and Mo5+ . For Mo6+ , the main photoelectron lines were positioned at 233.86 and 237.01 eV. The convolution of the Mo doublets proved that the Mo5+ chemical state existed in the as-synthesized material and they were located at 232.64 and 235.79 eV. These measured values are in a good agreement with the literature values. The narrow feature of the Mo6+ and Mo5+ with the 1.3 and 1.2 FWHM values suggested that Mo4+ and other possible Mo chemical oxidation states did not exist. It was proven from the deconvolution that the as-synthesized material contained ∼86.8 at% Mo5+ , while the remaining contained 13.2 at% Mo6+ . Sr2+ had well resolved spinorbit components with the value of 1.76 eV for 3d5/2 and 3d3/2 , as is the usual in its stable compounds. However, in addition to the welldefined 3d doublet, the shoulder formation was observed at the high energy side, which was not possible to be fitted into another doublet; therefore, this formation could be attributed to the low level electrostatic interaction with surface adsorbed oxygen. This discussion supports the formation of a pure SrMoO4 phase without any secondary SrO or SrCO3 . The relative amounts of cations were found to be 50.5 at.% and 49.5 at.% for Sr and Mo, respectively, which is close to the nominal stoichiometry of the as-synthesized compound. 3.3. SO2 sensor testing of the nano-SrMoO4 and post-mortem analysis All four morphologies of the SrMoO4 nanomaterials were tested using the same protocol described in the experimental section. In comparison to other morphologies of SrMoO4, the nano-flowers showed the highest sensor response at all three temperatures. The time dependent relative resistance change curves (R) of the nano-SrMoO4 at 600◦ C, 800◦ C and 1000◦ C are shown in Fig. 5(a–c). Nano-SrMoO4 showed a regular semi-conducting metal oxide behavior, where the relative resistance change was in accordance to the level of the target gas it was exposed. In this case, it can be assumed that the conduction band charge carrier concentration in the SrMoO4 was proportional to the partial pressure of the reducing gas in the sensor environment [71–73] Another significant point is that the nano-SrMoO4 showed an n-type response to the target gas at all concentration levels and testing temperatures. It was not observed that response switched from n-type to p-type (or opposite), as was previously observed for WO3 , MoO3 and NiMoO4 . The result revealed that nano-SrMoO4 showed an n-type response with the Rmax values of −6.7, −11.8 and −17.2 upon exposure to the 20 min pulse of 500, 1000 and 2000 ppm of SO2 at 600◦ C. The nano-SrMoO4 showed a smooth and repeatable response to the 5 min pulses of SO2 as well. At this temperature, the Rmax for the 5 min pulses of 500, 1000, 2000 ppm of SO2 were −6.3, −11.6, −17.1, respectively. Despite the detection at the 30 s pulse for the 2000 ppm SO2 (showing a Rmax of −3.7), the sensor could

Fig. 5. The relative resistance change (R) curves of the nano-SrMoO4 as a function of SO2 concentration at (a) 600◦ C (b) 800◦ C and (c) 1000◦ C.

not detect the SO2 at the 5 s and 15 s pulses. At 800 ◦ C, the nanoSrMoO4 showed the highest sensor response towards 2000 ppm SO2 among all other oxides tested. As can be seen from Fig. 5-b, the sensor signal was smooth and a signal was drift-free. In other words, the material recovered to its initial resistance once the target gas was removed from the testing chamber. The Rmax values were −6, −18 and −50.2 for the 20 min exposure of 500, 1000 and 2000 ppm SO2 at 800 ◦ C, respectively. At this temperature, the nano-SrMoO4 detected the 30 and 15 s exposures of 2000 ppm SO2 with the Rmax values of −6.6 and −3.9, respectively. Unfortunately, the sensor could not detect the 5 s pulse. The time dependent relative resistance change curve of the nano-SrMoO4 at 1000 ◦ C is presented in Fig. 5-c. The sensor material not only demonstrated very good repeatability, but also the sensing material permitted the ability to distinguish between the different concentrations levels of the target gas. The Rmax values were −23.5, −27.0, and −40.5 for the 20 min pulses of 500, 1000 and 2000 ppm SO2 , respectively. The same trend was valid for the 5 min pulses. In this case, the Rmax values are −17.9, −21.7, and −26.8 for 500, 1000 and 2000 ppm SO2 , respectively. At this temperature, the Rmax values for 30 and 15 s pulses were −15.0 and −11.3, respectively. The material showed very high sensor response and excellent repeatability (especially at 1000 ◦ C). The recorded response times for the nano-SrMoO4 against 2000 ppm of SO2 were 15.6, 12.0 and 13.2 min at 600◦ C, 800◦ C and 1000◦ C, respectively. These values were found to be lower than that measured in this work for the NiWO4 and SrWO4 compositions at 800 ◦ C and 1000◦ C. The only comparable work in literature to these results was completed by Morris et al. [11] They reported a 16.5 min response time at 375 ◦ C. Post-mortem assessment after SO2 testing included SEM and XPS analysis. Fig. 6 shows the overall microstructure and tortuosity of the nano-SrMoO4 powder after SO2 testing, while the inset figure provides a higher magnification SEM image revealing the nature of the intergranular connection. The micrograph shows that the nanomaterial succumbed to coarsening/sintering, and the nanofeatures were lost. The average grain size ranged from 1 to 5 ␮m. XPS analysis was completed on the nano-SrMoO4 after SO2 testing and the measured data is presented in Fig. 7. The sensor was cooled

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that the oxygen source is of prime importance in defining the steps for the sensing mechanism. Therefore, the concentration and state of the oxygen was quantified in detail in the previous section for the as-synthesized powder. A few of the more probable reactions defining the sensing mechanism are listed in Eq. (2) through Eq. (12). The overall redox reaction(s) that define the sensing mechanism include the reduction reaction of the oxygen species followed by the transformation of oxygen ions into regular lattice and/or interstitials sites (displayed in Eqs. (2)–(8)), and the subsequent oxidation of SO2 (and similarly for oxidation of H2 S, which will later be discussed) and release of SO3 from the surface (displayed in Eqs. (9)–(12)). O2(gas) ↔ O2(ads) −

Fig. 6. The microstructure of the nano-SrMoO4 after complete SO2 testing. Inset provides higher resolution.

O2(ads) + e ↔

(4)

O− + e− → O2− o (ads)

(5)

O− → O− i (ads)

(6)

O− + e− → O2− (ads) i

(7)

↔

O2− o −

(8)

SO2 + O → SO3 + e SO2 + O

2−

−

→ SO3 + 2e

−

(9) −

H 2 S + 3O → H 2 O + SO2 + 3e H 2 S + 3O

down in N2 atmosphere without any O2 background in order to preserve the final chemical composition. The Mo4+ oxidation state was detected at 231.9 eV in addition to Mo5+ and Mo6+ . The amount of each oxidation state was determined by deconvolution of the main Mo photoelectron position. The amount of Mo4+ , Mo5+ and Mo6+ were 38.6, 32.8 and 28.6 at%, respectively. The significant increase in Mo4+ was observed; this phase was not detected in the as-synthesized state. The increase in Mo4+ was accompanied by a decrease in Mo5+ concentration. The increase in FWHM values also suggested that non-stoichiometric sub-oxides formed in the nanoSrMoO4 , which were not present in the as-synthesized state. No metallic state for Mo was detected. It is important to confirm the overall stability of the composition, since many of the other W- and Mo-based compositions displayed significant chemical reduction after SO2 testing. Although it is still debatable about the exact mechanism that dictates the sensing for semi-conducting metal oxides toward reducing gases such as SO2 and H2 S [54], it is widely accepted in literature that the interaction of these semi-conducting oxides and the gas leads to a redox (reduction-oxidation) reaction that results in the release of electrons to the conduction band of the semiconductor. This reaction causes an increase in carrier concentration and a macroscopic decrease of the sensor resistance. The mechanism described above is valid for an n-type semi-conducting metal oxide with the grain size larger than that of the space charge layer depth [74]. This is a more holistic view of the electrical response; there is still minimal understanding of the redox reaction at the surface. The redox reaction depends upon the sulfur species adsorption, dissociation, surface diffusion, charge-transfer, and product desorption; this is to name just a few of the steps related to the sulfur species reaction. Similar reaction steps associated with the oxygen are just as important for the sensing mechanism, especially when source of the oxygen may vary due to the atmospheric conditions. The oxygen may originate from the atmosphere, and/or surface, bulk lattice, and interstitial sites within the sensing semi-conducting metal oxide. For the testing completed in this work, it is believed

(3)

O− + e− ↔ 2O− 2(ads) (ads)

O2− i

Fig. 7. Mo core photoelectron spectrum of nano-SrMoO4 after SO2 testing.

(2)

2O− 2(ads)

2−

(10) −

→ H 2 O + SO2 + 6e

−

(11) (12)

Before discussing the interrelation of each of the reaction steps in the sensing mechanism, the oxygen reduction must first be focused upon due to the nature of the testing conditions used in this work. The reduction reactions are believed to be of prime importance due to the high temperature and low oxygen content used in the current study (and related to the potential application discussed in the “introduction section”). The oxygen source described , O− ), bulk latin Eqs. (2)–(8) may originate from surface (O− 2(ads) (ads)

2− − tice (O2− O ) and interstitial (Oi , Oi ) sites within the nano-SrMoO4 . It is important to note that the functionality of typical low and room temperature semi-conducting metal oxide-based resistivetype sensors are highly dependent on the high concentration of adsorbed oxygen surface species, and the ability of regeneration of these species (and associated defect states) after steady state operation is achieved (or alteration of the reactant concentration). This dependence is drastically altered in the current work at high temperature, where the concentration of adsorbed oxygen would be insignificant. Hirschwald et al. indicated that the O− to O− 2(ads) (ads) transformation occurs at about 200◦ C, and desorption of the latter occurs beyond 250 ◦ C [73–77] Based on this fact, the functionality of the semi-conducting metal oxides theoretically should be terminated beyond 300◦ C; therefore, the number of semi-conducting oxide sensors discussed in literature that function beyond that temperature are limited. Many of the semi-conducting oxide sensors described in literature lose sensing capability due to desorption of the chemi/physi-adsorbed oxygen along with reduction of the sensing material at elevated temperature. Thus, for the functionality of the given nano-SrMoO4 sensing material used at high temperature, oxygen diffusion from lattice and interstitial sites are required for the redox reaction. It is known that scheelite oxides have the ability to include high concentrations of stable interstitial oxygen ions within its structure, and previous research has shown that this leads to a compensating mechanism to enhance structural stability under low oxygen partial pressure environments at high temperature [78–82].

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In the current work, the concentration of VO¨ and Oi as well as Mo5+ and Mo6+ were calculated by means of deconvolution of corresponding photoelectron positions and this data is presented in Fig. 4 (for the as-synthesized sensing material) and Fig. 7 (for the post-testing sensing material). From this data (and the previous discussion), it has been shown that the nano-SrMoO4 defect structure  contains both a high concentration of VO¨ and Oi , which assist in the overall redox reaction at elevated temperatures. This is especially important since the adsorption of O2 to the stoichiometric oxide surface would be very difficult compared to the non-stoichiometric counterpart [17] The oxygen vacancies play a very important role as oxygen sinks for atmospheric oxygen (expressed as Eqs. (2)–(8)) to replenish oxygen surface sites in order to retain the oxidation reactions (Eqs. (9)–(12)). The atmospheric oxygen may also be transferred into the interstitial and regular lattice sites (Eqs. (5)–(8)) within the structure to sustain the oxygen reaction (where the scheelite nano-SrMoO4 was shown to have a percentage of interstitial oxygen as characterized by the XPS data). It has been shown that these oxygen vacancies not only facilitate adsorption of molecular oxygen, but also SO2 gas at high temperature. This aspect is important since thermal desorption of the SO2 from metal oxide surfaces has been shown to increase at temperatures >500◦ C and most oxide surfaces are typically free of O2− and SO2 by 1000◦ C [18,83,84] The redox reaction is initiated with the SO2 interaction with the surface. As discussed above, it is assumed that the nano-SrMoO4 surface vacancy sites assist in trapping the sulfur species. Again, the O− and O2− is supplied to the surface reaction sites, as described above, where the SO2 (and similarly for H2 S) undergo an oxidation process as described by Eqs. (9)–(12). The release of the electrons (e− ) to the impurity level conduction band leads to the subsequent decrease in resistance. The sites that are exposed to SO2 do not form any compound with the cations, and thus, the possibility of further oxidation of SO3 to SO2− 4 may be ruled out. It is assumed that the final product (SO3 ) is released from the surface immediately at these testing temperatures, in accordance with that discussed in previous literature and the post-mortem XPS analysis completed in this work [18,71,73,83–87] It may be noticed in Fig. 5 that all the responses show a quick change in resistance followed by a slower exponential decrease in the response to the equilibrium level, which representative of a diffusion-controlled process. These two distinct responses indicate that the overall SO2 oxidation reactions (Eqs. (9)–(10)) are limited at the surface by the availability of the surface oxygen. The initial rapid response represents the reaction of the sulfur species with the available surface oxygen (such as the interstitial oxygen ions), and the second slower portion of the response is aligned with the change in kinetic rate due to the diffusion of oxygen to the surface reaction sites from the bulk and/or interstitial pathways within the scheelite structure. This diffusional limitation can be boldly seen in the difference in final response for the 5 and 20 min exposure times, where the diffusional time is too short for full oxygen diffusion from the bulk and interstitial pathways to complete the full oxidation of the surface sulfur species concentration. Therefore, the sensor response for the 5 min exposure never reaches full signal saturation due to the diffusional limitation.

3.4. Electrical characterization of micro- and nano-SrMoO4 sensing materials As shown in this work, the nano-SrMoO4 showed a high sensor response to SO2 , and this performance was quite repeatable. Interestingly, the sensing performance was even higher than that of its micron-size SrMoO4 counterpart. The micron- and nano-SrMoO4 shared the same nominal stoichiometry, and after testing at high

Fig. 8. Uv-Vis bandgap measurements for nano-SrMoO4 . Inset provides Uv-Vis measurement for micro-SrMoO4.

temperature, the grain size/morphology of the nano-SrMoO4 was similar to that of its micron-size counterpart. In order to provide a better understanding of the difference in sensing characteristics between the micron- and nano-SrMoO4 , the band gap and electrical resistivity (as a function of temperature) of each material was further measured and compared. The electronic contribution for the n-type sensor mechanism was studied using UV–Vis on the SrMoO4 materials. The formula proposed by Wood et al. [88] was used to estimate the bandgap of the nano-SrMoO4 (and micronsize SrMoO4 as a baseline) by extrapolating the linear portion of the UV–Vis curve. The UV–Vis absorption spectrum of the nanoSrMoO4 is presented in Fig. 8. The measured values for the band-gap were 3.7 and 3.9 eV for the nano-SrMoO4 and micron-SrMoO4, respectively. The band-gap value for the nano-sized powder was a good match with literature values [52,89–91] The decrease in the bandgap value can be attributed to the oxygen deficiency in the nano-SrMoO4 as compared to the micron-size counterpart. The absorption spectrum of nano-SrMoO4 displays the typical behavior for a wide bandgap semiconductor; however, it also should be noted that the distortion, which was not observed for micronSrMoO4 in the curvature of the higher energy section, is due to the Mo5+ defect states that create a continuous adsorption instead of a smooth cut-off. The total electronic resistance in the SrMoO4 is assumed to be governed by the resistivity of the bulk grain core and the grain boundary region (which includes the grain boundary Schottky barrier and double-layers near the grain boundaries). In this case, the large grains are thought to be separated by grain boundaries with a large neck formation. Therefore, the inter-grain electrical conduction would be governed by bulk trap limitations, while the absolute sensor resistance would be governed by the grain boundaries. The average grain size of nano-SrMoO4 , which is in the micron range (as can be seen in Fig. 6) is larger than that of the electron depletion layer which is in the range of a nanometer. Due to the coarsened grain structure of the nano-SrMoO4 at the temperature of interest, the conductance is assumed to be primarily limited by the Schottky barrier electronic conductance [74]. The magnitude of the Schottky barrier (EA ) can be estimated from the formula presented in Eq. (13). −(E A )

(E A )

G = P 0 e 2kB T ⇒ R = G 1− ⇒ R ∝ e 2kB T

(13)

The formula presented by Schierbaum et al. extended from conductance (G) to resistance (R), where a linear fit was applied to represent the temperature dependence of resistance [74] Fig. 9

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Fig. 10. The relative resistance change (R) curve for nano-SrMoO4 as a function of H2 S concentration. Fig. 9. Ln(R) vs. (kB T)−1 plot for the micro and nano-SrMoO4 .

shows the natural logarithm of the resistance data based on data acquisition for every other second versus (kB T)−1 for nano- and micro-SrMoO4 . The resistances of both porous SrMoO4 materials were measured in N2 atmosphere containing 1% O2 on the same sensor platform; a current of 5–10 ␮A DC current was applied to measure the resistance. Again, the plot shows the logarithmic fit of the actual data. By taking the slope of the ln(R) vs (kB T)−1 plot, it was possible to quantify the EA values for both materials. In Eq. (13), the EA represents the energy to form and move the charge carrier (e− ) through the Schottky barriers and grain boundary resistance, T represents absolute temperature, and kB is the Boltzmann constant, while P and  are the proportionality constant and bulk conductivity, respectively. The EA values calculated for nano-SrMoO4 and micro-SrMoO4 were 1.63 and 1.98 eV, respectively. Both values are higher than the values previously reported in the literature [92]. In this work, it is assumed that the Schottky barriers among the grains were formed due to the existence of interstitial oxygen ions and the trapping of electrons from the conduction band [93]. Nano-SrMoO4 undergoes significant coarsening/sintering starting at ∼600◦ C; it may be assumed that the grain size (or number of grain boundaries) did not significantly influence the sensing performance difference between the micro- and nano-SrMoO4 . The nano-SrMoO4 exhibited the lower band-gap and EA in comparison to the micro-SrMoO4 material. In addition, there was a slight difference in stoichiometry between the powders, where the micron-SrMoO4 showed a slight residual SrO phase which may have influenced the grain boundary state (and thus resistance). 3.5. SO2 sensor testing of nano-SrMoO4 at various oxygen levels Nano-SrMoO4 was tested for SO2 at 1000◦ C with the same testing protocol used throughout the current work at three different O2 concentrations (1, 5 and 20%). These experiments were completed in order to resolve the possible effect of fluctuations in oxygen partial pressure on the sensing mechanism under targeted operation environments (such as in the more highly oxygen-deficient areas within coal-fired power plants and gasifiers). It was concluded from the results (not included in this work) that a change in the oxygen concentration from 1% to 5% did not affect the Rmax more than 2%; however, a further increase from 1% to 20% in O2 concentration resulted in a 7.5% change in Rmax at the same temperature. The relatively low effect of oxygen partial pressure on the sensor signal is important to note. The possible fluctuations in the oxygen concentration could alter the oxygen vacancy concentration, which would alter the sensing mechanism significantly. Further

surface chemistry studies will be required to better understand the compensating mechanism for the altering localized oxygen concentration, and how this effects (or in this case, limits the effect) of the surface oxygen vacancy concentration. 3.6. H2 Sensor testing of the nano-SrMoO4 As previously discussed, semi-conducting metal oxide sensors for hydrogen sulfide (H2 S) detection have received much more attention than that for SO2 sensing. The main focus of these works related to safety sensors for low temperature (<500◦ C) applications. The human threshold exposure limit for H2 S is 10 ppm, with concentrations above 250 ppm H2 S causing instantaneous death [19] As discussed above, the nano-SrMoO4 showed high sensor response toward SO2 ; the same sensors were tested against H2 S. The following testing approach was employed, which was slightly altered from the protocol used for SO2 testing. This change was completed to better understand the sensing mechanism and to evaluate the sensor material under practical service environment, as well as, for potential use in fuel cell applications. The sensors were heated to 800◦ C at 5◦ C/min in pure N2 and then held at 800◦ C overnight without any oxygen background with data collection over the entire time. The total flow of all gases into the testing chamber was set at 100 sccm. The H2 S exposure was completed within a fuel gas blend containing 200 ppm of H2 S and balanced with H2 . The sensing material was tested for 5, 50 and 100 ppm of H2 S. A 1% O2 content was maintained during the H2 S exposure, as was the case for the SO2 testing. Fig. 10 shows the time dependent relative resistance change curve (R) of the nano-SrMoO4 against H2 S at 1000 ◦ C. The Nano-SrMoO4 showed a highly reliable sensor response toward H2 S under the exposure of H2 . The compound showed an n-type sensing response to the target gas at all concentration levels and testing temperatures. The response at 800◦ C (not included in this work) showed a difference between the exposure to 5, 50 and 100 ppm concentration levels of the target gas. Moreover, the difference in the gain of the normalized response increased at 1000◦ C. The material detected 5 ppm of H2 S with a Rmax value of −5. For the 20 min pulses for 50 and 100 ppm of H2 S, the Rmax values were −65 and −95, respectively. The nano-SrMoO4 showed a similar sensor response characteristic for 5 min pulses with a decrease in magnitude of the sensor response. The high Rmax and accurate results were obtained from the nano-SrMoO4 for the low H2 S impurities in H2 gas. Fig. 11 displays the XPS analysis of the nano-SrMoO4 sensor after H2 S testing. Fig. 11 also includes the XPS spectrum of the nano-SrMoO4 as-synthesized and post-testing in SO2 for compar-

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Fig. 11. XPS core spectrum of Mo after H2 S testing.

ison purposes. The sensor was cooled down in N2 without any oxygen background in order to preserve the final chemical state after testing. The Mo main peak positions and shapes suggested that there was a reduction of Mo6+ to Mo5+ and Mo4+ ; however, metallic Mo was not detected. There was no shift in core peak positions for Mo4+ and Mo5+ , but there was a 0.4 eV shift towards the low binding energy position observed for Mo6+ . The further broadening of the Mo4+ and Mo5+ peaks (exhibited by an increase in the FWHM values) suggests that additional sub-oxides were formed as was seen in the SO2 testing. The relative measured concentration of Mo4+ , Mo5+ and Mo6+ was 38.8, 48.5 and 14.2 at%, respectively. The relative amount of Mo4+ remained constant; however there was a further reduction in the quantity of Mo6+ , which accompanied by an increase in the amount of Mo5+ . During the 20 min exposure to 100 ppm H2 S, the measured low resistance value suggested that the composition was reduced to the metallic state which governed the electrical conductance [94–96]. It should be noted that the reduction of the scheelite SrMoO4 to a perovskite SrMoO3 has been reported by other researchers; therefore, we postulate that at 1000◦ C under the reducing atmosphere, the scheelite nano-SrMoO4 partially reduced to the perovskite SrMoO3 . The suggested reduction was proven by the detection of the Mo4+ , which corresponds to the oxidation state of the Mo in SrMoO3 . Interestingly, as soon as the H2 S/H2 gas mixture was removed from the testing environment, the nano-SrMoO4 recovered to its initial resistance value (indicating a reversible transformation) under 1% O2 balanced with N2 in 20 min. The 5 min pulses were also detected with full precision by the nano-SrMoO4 . Different researchers indicated that SrMoO4 could be reduced under high partial pressure of H2 at temperatures as high as 1200 ◦ C with oxygen background as low as (log P(O2 ) = −12.45). In these works, the SrMoO3 phase was found to be not stable and highly prone to oxidize back to SrMoO4 . The oxidation starts around 200◦ C [94–97]. 3.7. H2 Testing of the nano-SrMoO4 in coal syngas The high sensor response and accurate results obtained from the nano-SrMoO4 for H2 S prompted further testing in other operational conditions, such as the testing within coal syngas for sulfur impu-

Fig. 12. The relative resistance change (R) curves for the nano-SrMoO4 as a function of H2 S partial pressure at (a) 800◦ C (b) 1000◦ C.

rities. The coal syngas simulant used in this test was composed of 32% H2 , 40% CO, 20% CO2 , 1% O2 , 1% steam (H2 O) and 5% N2 with different levels of H2 S at 800 and 1000◦ C for 5 and 20 min pulses. The time dependent sensor response for the syngas composition is presented in Fig. 12-a and −b for 800 and 1000◦ C, respectively. During steady state exposure to the above-mentioned syngas composition, 4, 34 and 68 ppm of H2 S was introduced into the testing chamber. The nano-SrMoO4 unexpectedly showed a p-type sensing behavior. This is the first time nano-SrMoO4 showed n/p-type switch. The calculated Rmax values were up to 52 and 63 upon exposure to 64 ppm of H2 S for 20 min at 800◦ C and 1000◦ C, respectively. The sensor detected 4 ppm H2 S at 800◦ C, as well as, at 1000◦ C for the 5 min and 20 min pulses. In addition to the 20 min exposure, the compound managed to distinguish between the 4, 34 and 68 ppm H2 S concentrations in the syngas flow for the 5 min exposure time at both 800◦ C and 1000◦ C. The Rmax was reduced from 63 to 45 as the exposure time was changed from 20 min to 5 min at 1000◦ C. At this time, it is not possible to provide an explanation for this response switch in sensing behavior; however, it may be related to the complexity imposed by the CO and steam in addition to continuous presence of H2 in the testing chamber. As previously discussed, a p-type and n-type switch has been shown for various tungstate and molybdate compositions for certain temperature changes and reducing gas exposures. The bulk of the literature related to this anomaly was shown for the WO3 composition. Shimizu et al. observed the temperature dependent n/p-type switch upon exposure to SO2 ; however, the authors did not elaborate over the mechanism [39]. Fruhberger et al. indicated that the sensing mechanism was totally dependent on the H2 S and lattice oxygen interaction, and the subsequent desorption of the product (SO2 ). The work did not include a discussion on the chemical reaction between the surface adsorbed oxygen (interstitials as well) and H2 S. Barret et al. also reported the same characteristics for very low

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Fig. 13. H2 and CO cross-selectivity testing results for the nano-SrMoO4 .

H2 S partial pressures [98–100,27,101]. Further work is planned to investigate this change in response behavior. 3.8. Cross-selectivity testing of nano-SrMoO4 to H2 and CO Nano-SrMoO4 showed sensing to both SO2 and H2 S at the ppm level (1% O2 ) at ≥600◦ C. The altered sensing response of the nano-SrMoO4 within the coal syngas simulant instigated further study of H2 and CO sensing. The cross-selectivity tests towards H2 and CO were completed in accordance with the testing procedure described previously for SO2 . The comparison of the Rmax values for a 20 min exposure to 4000 ppm of H2 and CO balanced with N2 (with 1% O2 ) at 600, 800 and 1000 ◦ C are presented in Fig. 13 for nano-SrMoO4 . It should be noted that the concentration levels were doubled for CO and H2 in comparison to SO2 (where these tests were conducted for ≤2000 ppm level). As seen in Fig. 13, the material showed the typical n-type response with very low sensor response towards H2 . The Rmax for the 4000 ppm H2 was approximately −10 and −5 at 800 and 1000◦ C, respectively. This level of sensor response is far below the response values measured for SO2 at the same temperature; this may indicate that cross-selectivity may not be a significant concern. The Rmax for CO at 1000◦ C was −2.5, which was half of the Rmax measured for H2 at the same temperature. The sensor showed no response to CO at 600 ◦ C. At 800◦ C and 1000◦ C, the measured Rmax for nano-SrMoO4 against SO2 was 50 and 16 times higher, respectively, than that of CO at the same temperatures. The Rmax values for the 20 min exposure of 2000 ppm of SO2 are presented for comparison purposes. 4. Conclusion An array of thick-film sensors based on micro-size tungstate and molybdate powder compositions were initially screened for SO2 sensing material on a chemiresistive platform at 600–1000◦ C. For many of these compositions, the testing performance presented in this work was the first reported in literature. Most of the compositions showed intermittent response to the 2000 ppm SO2 at 600–800◦ C, but then the compositions reduced to various suboxide or metallic states as the temperature was raised to 1000◦ C. The SrMoO4 showed the highest sensor response and stability within the entire temperature range, thus the compound was synthesized by hydrothermal method to the nanoscale (in order to increase the active surface area, and thus magnify response). The nano-SrMoO4 showed high sensor response for the 500–2000 ppm SO2 , and the response was found to be very reliable and repeat-

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able. The nano-SrMoO4 was further tested in 100 ppm of H2 S diluted in H2, as well as, in a coal syngas composition. The material again showed very promising sensing capabilities and low cross-selectivity under the extremely reducing atmosphere and high temperatures. The high sensor response values, and stable and smooth response characteristics, were attributed to the high con centration of oxygen vacancies (VO¨ ) and oxygen interstitials (Oi ), which were characterized by XPS. The presence of these two defect states were presented as possible structural characteristics that helped maintain sulfur specie adsorption and oxidation at the elevated temperatures. This was further supported by the difference in sensor response kinetics between the micron-SrMoO4 and nanoSrMoO4 , where the final microstructure of each were similar after testing. Further work is required to better understand the change in the sensing mechanism (n/p-type switch) for the SrMoO4 when testing for syngas composition, in addition to this, further surface chemistry studies are required under varying O2 concentrations to obtain comprehensive understanding regarding the effect of the defect chemistry on the compound‘s sensing mechanism. Acknowledgement This work was supported by the Department of Energy under Award Number DE-FE0003872. We acknowledge the use of the WVU Shared Research Facilities. The authors also would like the acknowledge Dr. Wei Ding and Mr. Chuck Coleman for their assistance. References [1] EPA, United State Environmental Protection Agency, 14 09 2015, http:// www3.epa.gov/airquality/sulfurdioxide/. [2] C. Powell, B. Morreale, MRS Bull. 33 (2008) 309–315. [3] Annual Energy Outlook 2007 with Projections to 2030, U.S. DOE, Washington, DC, 2007. [4] H.F. Graf, J. Feichter, B. Langmann, J. Geop. Res. 102 (D9) (1997) 10727–10738. [5] US EPA, 14.09.2015. [Online] Available: http://www3.epa.gov/airquality/ sulfurdioxide/. [Accessed 18.11.15]. [6] Z. Darmastuti, C. Bur, P. Moller, R. Rahlin, N. Lindqvist, M. Andersson, A. Schütze, A. Lloyd Spetz, Sens. Actuators B: Chem 194 (2014) 511. [7] S.M. Kanan, O.M. El-Kadri, I.A. Abu-Yousef, M.C. Kanan, Sensors 9 (2009) 8159–8196. [8] D.L. West, F.C. Montgomery, B.L. Armstrong, Sens. Actuators. B: Chem. 162 (2012) 409–417. [9] D. Shin, T.M. Besmann, B.L. Armstrong, Sens. Actuators B: Chem. (2012). [10] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Crit. Rev. Solid State Mater. Sci. 29 (2010) 111–188. [11] D. Morris, R.G. Egdell, J. Mater. Chem. 11 (2001) 3207–3210. [12] X. Liang, T. Zhong, B. Quan, B. Wang, H. Guan, Sens. Actuators B: Chem. 134 (2008) 5–30. [13] A. Neubecker, T. Pompl, T. Doll, W. Hansch, I. Eisele, Thin Solid Films 310 (2016) 19–23. [14] C. Fujimoto, Y. Hayakawa, A. Ono, Sens. Actuators B: Chem. 19 (1996) 1–194. [15] S.C. Lee, S.Y. Kim, B.W. Hwang, S.Y. Jung, D. Ragupathy, I.S. Son, D.D. Lee, J.C. Kim, Sensors 13 (2013) 3889. [16] E.B. Varhegyi, J. Gerblinger, F. Reti, I.V. Perczel, H. Meixner, Sens. Actuators B: Chem. 24 (1995) 631–635. [17] E.B. Varhegyi, J. Gerblinger, F. Reti, I.V. Perczel, H. Meixner, Sens. Actuators B: Chem. 24–25 (1995) 631–635. [18] D.H. Dawson, G.S. Henshaw, D.E. Williams, Sens. Actuators B: Chem. 26–27 (1995) 76–80. [19] S. K. Pandey, K. H. Kim, K. T. Tang, Vol (2012), 87–99. [20] G. Fang, Z. Liu, C. Liu, K. Yao, Sens. Actuators B: Chem. 66 (2000) 46. [21] Z. Sun, H. Yuan, Z. Liu, B. Han, X. Zhang, Adv. Mater. 17 (2005) 2993. [22] U. Kersen, L. Holappa, Appl. Phys. A 85 (2006) 431–436. [23] C.V. Gopal, S.V. Reddy, S.V. Manorama, V.J. Rao, J. Mater. Sci. Lett. 70 (2000) 775. [24] G.N. Chaudhari, M.A. Alvi, H.G. Wankhadea, A.B. Bodade, S.V. Manorama, Thin Solid Films 520 (11) (2012) 4057. [25] S.V. Jagtab, V. Kadu A, V.S. Sangawar, S.V. Manorama, G.N. Chaudhari, Sens. Actuators B: Chem. 131 (2008) 290. [26] G.N. Chaudhari, D.R. Bambole, A.B. Bodade, P.R. Padole, J. Mater. Sci. 41 (2006) 4860. [27] E.P. Barret, G.C. Georgiades, P.A. Sermon, Sens. Actuators B: Chem. 1 (1990) 116.

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Biographies Engin C¸iftyürek received his B.S. degree in physics engineering (2006) and M.S. degree in materials science and engineering (2009) from Istanbul Technical University (ITU). He obtained his Ph.D. degree (2014) in mechanical engineering at WVU. Katarzyna Sabolsky received her B.S. degree in ceramic engineering from Alfred University, Alfred, NY, in 1997, and her M.S. degree in material science and engineering from University of Florida, Gainesville, FL in 1999. She is currently an Associate Engineering Scientist at West Virginia University within the Mechanical and Aerospace Engineering Department. Her research interests include hightemperature sensors, solid-state lithium ion batteries and solid oxide fuel cells. Edward M. Sabolsky received his B.S. in ceramic science and engineering with a minor in metallurgical engineering from The Ohio State University in 1996. He completed his Ph.D. in materials science and engineering at Pennsylvania State University in 2001. He joined NexTech Materials, Ltd. (NexTech) in 2001, where he continued his work in ceramic powder synthesis, forming, and thermal processing of piezoelectric/dielectric, sensor, catalyst, and solid-oxide fuel cell (SOFC) materials. In 2006, He assumed the position of Fuel Cell Group Leader at NexTech, where he managed the SOFC research group. He joined the Fine Ceramics and Igniter R&D Group within the Saint-Gobain High-Performance R&D Center in 2007 as a Senior Research Scientist. He finally moved to West Virginia University in 2008 where he currently holds the position of Associate Professor, and continues his work in advanced ceramic processing and materials development for energy-related applications.